Popular beat combo as a synonym for "pop group" is a cliché phrase within British culture. It may also be used more specifically to refer to The Beatles, or other purveyors of beat music.
The deliberately out-dated phrase may be used as a tongue-in-cheek synonym, or by someone to denigrate a pop group referred to, or may be used of another person's views to imply that they are "out of touch". It may also be used to ridicule legalese and antiquated courtroom practices.
The phrase is frequently used in the BBC panel game Have I Got News For You, making fun of Ian Hislop's supposed lack of knowledge about modern music.
Derivation
It is widely held that the phrase "popular beat combo" was coined in an English courtroom in the 1960s by a barrister in response to a judge's query (for the benefit of the court's records) as to who "The Beatles" were; the answer being "I believe they are a popular beat combo, m'lud." However, this attribution has never been verified, and remains the stuff of urban legend, despite the efforts of Marcel Berlins, legal correspondent for The Guardian newspaper, to track it down.
The phrase may have been influenced by events in the 1960 obscenity trial of Lady Chatterley's Lover in which the legal profession was ridiculed for being out of touch with changing social norms when the chief prosecutor, Mervyn Griffith-Jones, asked jurors to consider if it were the kind of book "you would wish your wife or servants to read".